Elastomeric sealants are frequently used in the field of semiconductors, but they are often unsuitable for use in areas where high heat resistance is required. The elastomeric sealants are permeable and cause leakage, and therefore their sealing properties are insufficient when high airtightness is required.
Metal gaskets are sealants having high airtightness and high heat resistance. The metal gaskets, however, entail a high clamping force to ensure high sealing properties, which leads to increase in size and thickness of vessels or apparatuses where the gaskets are attached.
Of the metal gaskets, spring-energized metal C-rings provide high sealing properties with a relatively low clamping force. However, the clamping force is still ten times or more as high as that of rubber O-rings.
The following methods (1) and (2) are suggested to solve such problems.
(1) The sealing surface of a metal gasket is processed for easy stress concentration, whereby high stress is generated even at a small load (clamping force) and good conformability is obtained.
For example, JP-U-2000-10 (Patent Document 1) teaches that the sealing surface has protrusions (numeral 18 in FIG. 2). However, gaskets according to this teaching require a very high clamping force, and this problem cannot be solved even by using rubbers in combination therewith.
Japanese Patent No. 3110307 (Patent Document 2) describes that the sealing surface has dents (grooves). However, the corners of the grooves come in contact with mating parts.
As described above, a relatively high clamping force is required to deform metals and create an effective seal even with the stress concentration according to the method (1).
(2) A metal gasket is provided with a soft covering layer (coat, plating).
JP-A-H09-32928 (Patent Document 3) discloses that a polyimide resin coat is formed on a metal gasket.
According to the method (2), however, the soft covering layer is preferentially deformed at a low stress and the sealing surface of the metal gasket is not deformed easily. Accordingly, stable sealing properties cannot be reliably obtained at a low stress.
Although the methods (1) and (2) have some effects as described above, even the combination of the methods (1) and (2) needs a very high clamping force to achieve a desired high seal, for example more than several times as high as the clamping force required for O-rings. Thus, there has been a demand for sealants achieving high airtightness at a lower clamping force.    Patent Document 1: JP-U-2000-10    Patent Document 2: Japanese Patent No. 3110307    Patent Document 3: JP-A-H09-32928